All About Circuits Forum Limiting AC Current
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

#1
05-10-2009, 09:13 PM
 StarfleetRP Junior Member Join Date: Mar 2009 Posts: 24
Limiting AC Current

Hi, I have been looking around for ways to limit the current flowing through a circuit to a maximum value. I have only been able to find ways of doing this for DC circuits, however this is an AC circuit. I would like to limit a 25V 4A AC transformer to only have an output of 250mA (give or take). I would appreciate any advice you have on how I could accomplish this.
#2
05-10-2009, 10:27 PM
 studiot E-book Developer Join Date: Nov 2007 Location: Somerset UK Posts: 4,014

The simplest way is probably to use a second 1:1 transformer, which can only supply .25 amps.
__________________
Do I look old?
I don't feel old.
I don't feel anything till noon.
Then it's time for my nap.
#3
05-11-2009, 04:31 AM
 StarfleetRP Junior Member Join Date: Mar 2009 Posts: 24

Sorry I forgot to mention that I did find a solution using a secondary transformer, but that would not be suitable for the needs of this project. However thanks for the help so far!
#4
05-11-2009, 05:31 AM
 Suzkuz New Member Join Date: May 2009 Posts: 2

A resistor?
A fuse?
An ohmicly biased fet in parallel with the load that sinks a proportionate amount of current, so the load itself never sees more then 250mA?
#5
05-11-2009, 09:08 AM
 leftyretro Senior Member Join Date: Nov 2008 Location: Hercules, Ca. (SF Bay Area) Posts: 394

Quote:
 Originally Posted by studiot The simplest way is probably to use a second 1:1 transformer, which can only supply .25 amps.
And then what, it burns up it's windings to maintain the limit? That is not a well thought out 'solution'.

A true automatic current limit, where the unit starts to drop it's output voltage by the amount nessesary to maintain the .25amp maximum current, would need to work just like current limiting circuits for DC, but would required to measure and control both the positive and negitive portion of the sine wave. Some kind of direct coupled AB amp stage with current sensing/foldback or possibly a modified H-drive circuit with current limiting could be made to work.

Lefty
__________________
Measurement changes behaviour

Last edited by leftyretro; 05-11-2009 at 09:16 AM.
#6
05-11-2009, 12:27 PM
 studiot E-book Developer Join Date: Nov 2007 Location: Somerset UK Posts: 4,014

Quote:
 And then what, it burns up it's windings to maintain the limit? That is not a well thought out 'solution'
In my bathroom there is a shaver socket, protected by just such a transformer. It has never burnt out or burnt anything else yet.
__________________
Do I look old?
I don't feel old.
I don't feel anything till noon.
Then it's time for my nap.
#7
05-11-2009, 12:34 PM
 beenthere Senior Member Join Date: Apr 2004 Location: Missouri, USA (GMT -6) Posts: 15,815 Blog Entries: 10

It's not practical because the technology is obsolete, but a saturable reactor would be ideal for AC current limiting. Here's the Wikipedia article - http://en.wikipedia.org/wiki/Saturable_reactor
__________________
First comes the hardware, then the software.
#8
05-11-2009, 08:45 PM
 CDRIVE Senior Member Join Date: Jul 2008 Location: S. Florida USA Posts: 2,223

Quote:
 Originally Posted by Suzkuz A resistor? A fuse? An ohmicly biased fet in parallel with the load that sinks a proportionate amount of current, so the load itself never sees more then 250mA?
Ouch! What are you thinking?
__________________
__________________________________________________ ___________________________
Disclaimer: Any information that I post is intended for educational purposes only. No warranty or liability is expressed or implied. Note: Working with household line voltages (Mains) can be dangerous. Proceed at your own risk!
#9
05-11-2009, 08:54 PM
 Externet Senior Member Join Date: Nov 2005 Location: Mideast USA Posts: 473

Connect a 115V light bulb in series to the primary of the transformer; choosing its wattage until the current does not exceed 250mA.
Or,
Connect a 24v light bulb in series with the 24VAC secondary;
choosing its wattage until the current does not exceed 250mA.
A 12V automotive bulb may work too.

Miguel
__________________
Abolish the deciBel !
#10
05-11-2009, 09:20 PM
 CDRIVE Senior Member Join Date: Jul 2008 Location: S. Florida USA Posts: 2,223

Quote:
 Originally Posted by leftyretro A true automatic current limit, where the unit starts to drop it's output voltage by the amount nessesary to maintain the .25amp maximum current, would need to work just like current limiting circuits for DC,..... Lefty
I was thinking along the same lines and was going to spice it. I quickly aborted because of the OP's transformer limitations. If his load requires 25VAC and the transformer is 25VAC then any solid state solution would not deliver the full 25VAC.

StarFleet, how much voltage drop can you tolerate?
__________________
__________________________________________________ ___________________________
Disclaimer: Any information that I post is intended for educational purposes only. No warranty or liability is expressed or implied. Note: Working with household line voltages (Mains) can be dangerous. Proceed at your own risk!

 Tags current, limiting

 Related Site Pages Section Title Worksheet Step-up, step-down, and isolation transformers Textbook Rectifier/filter circuit : Discrete Semiconductor Circuits

 Similar Threads Thread Thread Starter Forum Replies Last Post Mad Professor General Electronics Chat 11 06-06-2009 07:18 PM dude521 General Electronics Chat 13 04-23-2009 06:45 PM Bill_Marsden General Electronics Chat 13 02-06-2009 07:03 PM damianhealey The Projects Forum 7 09-25-2008 11:17 PM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 03:41 PM.